1. Field of the Invention
This invention relates to a flat panel speaker which employs a plate of a transparent piezoelectric material.
2. Description of the Prior Art
Recently, the speech synthesizer has proceeded in micro electronic devices such as melody speaking wrist watches and electronic micro-calculator. In the electronic wrist watch, multifunctional factors have been required because the expression of a time is not limited to a visual one but a specified time needs to be aurally expressed as an alarm sound or the like.
By way of example, the conventional construction of a digital electronic timepiece of this type consists of an optical dial which is provided with a character display or second-hand (moving needles) display, a metallic body which supports the watch module, a piezoelectric element which is installed in the body, and a front glass plate which is stuck to be unitary with the piezoelectric element. The glass plate with a piezoelectric element is resonated in a predetermined frequency band by applying an electric alarm signal, and the vibration generates a buzzer sound, melody or the like. In the buzzer for the watch, the sound of any specified frequency within a frequency range of 2-4 KHz is selected, and the resonator is excited at its single resonance frequency in order to produce the sound at the lowest possible voltage. Thus, as the resonance frequency of the resonator is as simpler as can be, (that is, Q becomes higher), the efficiency becomes higher, so that the disk shape resonator has been used in practice and free from subresonances etc.
Such buzzers for watches are disclosed in Japanese Patent Application Laid-open Specification No. 55171/1978, etc.
Further, there has recently been proposed a watch which is endowed with the function of generating, not only the buzzer sound, but also a melody sound. These digital watches which appeal to the ear are also used for the drivers of running cars and bicycles and for the visually handicapped.
The buzzer sound, however, has been disadvantageous in that since originally it is intensely felt as an alarm or an emergency sound, it promotes a psychological restlessness more than is necessary, so it is not accepted as a pleasant sound. In order to change the sound quality so as to bring the buzzer sound close to the human voice, bulky accessory circuits including a speech synthesizer are required. This measure is considered impossible for small-sized electronic appliances such as the digital watch.
As another example of the prior art, there has been an idea according to which a voice producing source such as subminiature speaker is intended to be contained inside the body of a watch. Since, however, the watch originally requires hermetic sealing for water-proof etc., the idea is undesirable in point of disposing a perforated portion for emitting sounds. Furthermore, the small-sized electronic appliances such as watches require decorative factors. Especially the installation of an accessory component for another function onto the dial side spoils the sense of beauty and is demeritorious commercially. This has led to the disadvantage that a space for installing sound producing means is limited still more.
Further, many of electronic computers and devices for education etc. have recently been provided with a speech synthesizer, that is, micro talking devices, which produces human voices. These appliances are generally driven with batteries, and are desired to be small in size and light in weight. In this regard, a speaker portion occupies a large space and is therefore desired to be miniaturized. A miniature speaker, however, has had the disadvantage of an inferior sound quality.